The present invention relates to an optical semiconductor device in which a photodetector and a semiconductor laser element are formed on a single substrate and a method for fabricating the same.
A light emitting element and a photodetector are elements for mutual conversion between an optical signal and an electric signal, and are employed in various kinds of art field. In the field of optical disks such as CDs (Compact Discs) and DVDs (Digital Versatile Disc), they are main devices in optical pickups for reading/writing signals recorded on an optical disks.
In recent years, in accordance with demand for high performance and high integration, a photodiode serving as a photodetector and various electronic elements such as a bipolar transistor, a resister, a capacitance, are formed on a single substrate to compose a so-called opto-electronic integrated circuit (OEIC) device. For further size reduction and higher integration, OEIC devices are widely used in which a semiconductor laser element as a light emitting element and a micro mirror for changing a light path of the laser beam output form the semiconductor laser element are mounted. The OEIC devices of this kind are generally formed by a bipolar transistor fabricating method. In addition, the OEIC devices are required to include both a photodetector having high photosensitivity, high-speed operability and low noise characteristics and a high-speed, highly accurate bipolar transistor.
A conventional optical semiconductor device will be described below with reference to the drawings.
FIG. 10 shows schematically a sectional construction of an optical semiconductor device, that is, an OEIC device according to the conventional example. As shown in FIG. 10, a N-type epitaxial layer 102 is formed on a principal surface of a semiconductor substrate 101 made of P-type low impurity concentration silicon.
In the semiconductor substrate 101 and the N-type epitaxial layer 102, a transistor section 200 composed of a NPN bipolar transistor, a photodetector section 220 composed of a PIN photodiode and a light emitting element section 240 including a semiconductor laser chip 125 are formed to compose the OEIC device.
The transistor section 200, which is a two-layer polysilicon self aligned type NPN transistor, is composed of: a high concentration N-type emitter region 106; a P-type base region 107 formed below the emitter region 106; a collector region 108 made of the N-type epitaxial layer 102 and formed below the base region 7; a high concentration N-type collector buried region 109 formed below the collector region 108; an emitter electrode 110 formed above the emitter region 106; a base electrode 111 connected electrically to the peripheral portion of the base region 107; and a collector electrode 112 formed above the collector buried region 109 and connected electrically to the end portion of the collector buried region 109.
The light receiving section 220 is composed of: a cathode layer 115 made of the N-type epitaxial layer 102; a high concentration N-type cathode surface layer 116 formed on the cathode layer 115; a high concentration N-type cathode contact layer 117 formed around the cathode surface layer 116; and a cathode electrode 118 formed above the cathode contact layer 117.
In each of the transistor section 200 and the photodetector section 220, an isolation oxide film 113 for electrically isolating the elements is formed by local thermal oxidation, that is, so-called LOCOS. A high concentration P+-type isolation layer 114 is formed below the isolation oxide film 113.
In the photodetector section 220, the P+-type isolation layer 114 located in the peripheral portion of the photodetector section 220 in the semiconductor substrate 101 functions as a part of an anode and is connected electrically to an anode electrode 120 with the intervention of a high concentration P-type anode contact layer 119 formed on the P+-type isolation layer 114. A portion of the low concentration P-type semiconductor substrate 101 located below the cathode layer 115 serves as an anode region, and is taken outside as a current from the anode electrode 120 through the P+-type isolation layer 114 and the anode contact layer 119. On the cathode surface layer 116 serving as a light receiving face, an anti-reflection film 121 is provided for reducing reflection of incident light 122 on the cathode surface layer 116.
In the light emitting element section 240, a micro mirror region 123 is formed which is formed of a trench formed by digging the N-type epitaxial layer 102 and the upper part of the semiconductor substrate 101 by anisotropic etching. On the bottom face of the trench, a semiconductor laser chip 125 is fixed with the intervention of a laser lower electrode 128, a laser wire 127 and a protection film 126. The laser wire 127 is lead outside the trench along the wall face from the bottom face of the trench. The protection film 126 is formed so as to cover each upper face of the transistor section 200 and the photodetector section 220.
As shown in FIG. 10, laser light emitted from a side facet of the semiconductor laser chip 125 is reflected on the surface of the micro mirror region 123 to be output in a direction approximately perpendicular to the principal surface of the semiconductor substrate 110.
The operation of the thus composed OEIC device will be described below.
Application of a current over a threshold value to the semiconductor laser chip 125 causes induced emission and oscillation, so that coherent laser light 129 is output in a direction parallel to the principal surface of the semiconductor substrate 101. In the case where the micro mirror region 123 forms an angle at 45 degrees with respect to the substrate surface, the emitted laser light 129 is reflected on the surface of the micro mirror region 123 to rise in a direction perpendicular to the substrate surface. The reflected laser light 129 is irradiated on, for example, an optical disk or the like and a part of the thus reflected light becomes incident light 122 to enter in the photodetector section 220.
The incident light 122 that enters in the photodetector section 220 is absorbed in the semiconductor substrate 101 serving as the anode and the cathode layer 115 to generate electron hole pairs. When reverse bias voltage is applied to the photodetector section 220 at that time, a depletion layer is extended toward the semiconductor substrate 101 where impurity concentration is low. The electron hole pairs generated in the extended depletion layer and the vicinity thereof diffuse and drift separately so that the electrons and the holes reach the cathode contact layer 117 and the anode contact layer 119, respectively, thereby generating a photocurrent. Upon receiving the thus generated photocurrent, an electronic circuit composed of a NPN transistor, a resistor, a capacitor and the like performs predetermined amplification and signal processing to output the photocurrent as a recording or replay signal of an optical disk.
As described above, in recent years, in optical semiconductor devices having photodetector for optical pickup used in CDs and DVDs, high photosensitivity, high-speed operability and downsizing are strongly demanded in association with high-speed driving of optical disks and increasing density of recorded signals.
In the aforementioned conventional optical semiconductor device, however, the photocurrent generated from incident light is divided to the diffusion current component and the drift current component as described above, wherein the diffusion current component is dominant diffusion that the minority carriers move up to the end portion of the depletion layer. For this reason, the response speed of the diffusion current component is lower than the drift current component drifting by the electric field in the depletion layer, which is a factor of deterioration the frequency characteristic of the photodetector section 220 made of a photodiode.
Especially, infrared light used in CDs, which has a small absorption coefficient to silicon, reaches deep inside of the semiconductor substrate 101 and carriers generated at the deep part contributes to the current, which restrict high-speed operation. In this connection, it is impossible to form the photodetector section 220 and the light emitting element section 240 integrally on a single semiconductor substrate 101 for exhibiting high photosensitivity and high-speed operation.